Breathing assistance device, and method of regulation

ABSTRACT

A breathing assistance device having: a turbine for generating flow of pressurized respiratory gas, a duct for carrying pressurized gas to a patient, a mechanism to control gas pressure capable of elaborating a pressure setting for the turbine, the turbine is connected to a speed sensor for acquiring a signal corresponding to the rotation speed of a rotating element of the turbine, and the mechanism of controlling the calculation linked to the speed sensor in order to elaborate using the signal, a pressure setting sending the pressure setting to the turbine. Another aspect of the invention relates to a method for regulating the pressure of a respiratory gas delivered by a turbine to a patient, the method involving elaborating a pressure setting for the turbine, characterized in that the pressure setting is elaborated using a signal representative of the rotation speed of a rotating element of the turbine.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national stage application ofInternational Application No. PCT/IB2004/000898, filed on Jan. 27, 2004,which claims the benefit of French Application No. 0300858, filed onJan. 27, 2003, and of U.S. Provisional Patent Application No.60/496,074, filed on Aug. 18, 2003.

This invention relates in general to methods and devices for breathingassistance.

More precisely, the invention relates to a breathing assistance devicecomprising:

-   -   a turbine to generate a flow of pressurised respiratory gas,    -   a duct to carry the pressurised gas to a patient,    -   means for controlling the gas pressure capable of elaborating a        pressure setting for the turbine.

And the invention also relates to a method for regulating the pressureof a respiratory gas delivered by a turbine to a patient, the methodconsisting of elaborating a pressure setting for the turbine.

Devices of the type mentioned above already exist.

The basic architecture of such devices is shown in a very schematicmanner in FIG. 1 a, which shows a device 10 a consisting of a turbine100 a allowing to generate a flow of pressurised respiratory gas (air orother), a means 120 a allowing a patient to receive said pressurised gasand a duct 110 a to carry the gas from the turbine 100 a to means 120 a.

Means 120 a is generally made up of a mask that can include vents tomake respiratory gas leaks possible.

Note that this means can be replaced with an expiratory valve. And theinvention applies to devices with leakage masks as well as to expiratoryvalve devices.

Note that the term “patient” is here used broadly, and does notnecessarily correspond to a person afflicted with an extensivepathology.

The devices according to the invention can thus be implemented fornumerous applications, for example to provide respiratory assistancewhile a patient is sleeping with a view to treating sleep apnea.

However, the devices according to the invention are not in any waylimited to treating sleep apnea, which is an application of theinvention mentioned here by way of non-limitating example.

The invention relates to in fact as we shall see a new type of deviceand associated regulation, and its applications are extremely broad.

Returning to the device of the state of the art shown in FIG. 1 a, suchdevices are known to be implemented by controlling the pressuregenerated by turbine 100 a in such a way that this pressure has a fixedvalue.

Such devices are said to be of the continuous pressure airway pressuretype (and are generally known under the acronym of CPAP—Trade Mark—inEnglish).

These devices may however not be accepted by a portion of the patients,or be improperly adapted for treating certain pathologies.

More elaborate devices also exist, in which the means of controlling gaspressure are capable of elaborating several different pressure settingsfor the turbine.

Such devices are shown (here again in a very schematic manner) in FIG. 1b.

This figure shows a device 10 b. On this figure the elements labeled as100 a, 110 a and 120 a are the same as those elements in FIG. 1 a.

Device 10 b further comprises a flowmeter 130 b capable of providing tomeans of calculation 132 b a measurement of flow in duct 110 a.

Means of calculation 132 b are capable of elaborating, as a function ofthe measured flow, a pressure setting that will be sent to the turbine(or more precisely to a turbine operation regulation circuit) via aconnection 131 b.

This disposition makes it possible to adapt the pressure according tothe flow measured in duct 110 a, with this flow being linked to therespiratory activity of the patient.

The start of inspiration or expiration of the patient can thus inparticular be detected, and the pressure setting sent to the turbine canbe adapted according to the cycle (inspiration or expiration) which isin progress or which is starting.

EP 425 092 shows an example of such a device.

These devices correspond to an enhancement compared to devices of thetype as shown in FIG. 1 a.

But they require a flowmeter to be integrated.

Such a flowmeter is an expensive element. It furthermore has a tendencyto complicate the device and to increase breakdown risks of it as wellas increase its cost.

The purpose of the invention is to further enhance the devices of thetype mentioned at the beginning of this text, by avoiding theinconveniences and limitations exposed above.

In order to reach this purpose, the invention offers according to afirst aspect a breathing assistance device comprising:

-   -   a turbine to generate a flow of pressurised respiratory gas,    -   a duct to carry the pressurised gas to a patient,    -   control means for controlling the gas pressure capable of        elaborating a pressure setting for the turbine,

characterised in that the turbine is associated to a speed sensorcapable of acquiring a signal corresponding to the rotation speed of arotating element of the turbine, and the control means include means ofcalculation connected to said speed sensor in order to elaborate fromsaid signal a pressure setting and send said pressure setting to theturbine.

Preferable but not limited aspects of such a device are the following:

-   -   said speed sensor implements a Hall effect sensor,    -   said speed sensor is a sensor capable of acquiring a speed        signal from the turbine linked directly to the rotation speed of        a rotating element of the turbine,    -   the means of calculation elaborate the pressure setting        according to variations in speed,    -   said means of calculation are capable of detecting new        inspiration or expiration cycles, and to consequently adapt the        level of the pressure setting,    -   said means of calculation are associated to a program for        detecting inspiratory cycle using a comparison between:        -   A memorised speed value extrapolated using recent values of            measured speed, and        -   An instantaneous speed actually measured,    -   said means of calculation are associated to a program for        detecting inspiratory cycle using a comparison between:        -   A memorised speed value as representative of a recent speed            bearing, and        -   An instantaneous speed actually measured,    -   said means of calculation are associated to a program for        detecting inspiratory cycle using a comparison between:        -   A memorised speed value as representative of a speed at the            end of an expiratory cycle, and        -   An instantaneous speed actually measured,    -   said means of calculation are associated to several programs for        detecting inspiratory cycle operating simultaneously, and are        capable of elaborating a pressure setting corresponding to a        start of inspiration as soon as one of said programs for        detecting inspiratory cycle has signalled a start of        inspiration,    -   the programs(s) for detecting inspiratory cycle is (are)        associated to disabling means for a determined duration        following the start of a new expiratory cycle,    -   the means of calculation are associated to a program for        detecting expiratory cycle,    -   said program for detecting expiratory cycle uses a comparison        between:        -   A maximum speed of the turbine memorised, corresponding to a            cycle of inspiration, and        -   An instantaneous speed actually measured,    -   said means of calculation include a microprocessor connected to        the speed sensor and an input for pressure setting of the        turbine,    -   the device also includes a pressure regulation loop comprising:        -   a pressure sensor on the duct, and        -   a circuit receiving the pressure setting resulting from the            means of calculation as well as the pressure measured by the            pressure sensor, said circuit being capable of elaborating            an instantaneous setting for turbine rotation speed, said            circuit being connected to an input for speed setting of the            turbine.

The invention offers according to a second aspect a method forregulating the pressure of a respiratory gas delivered by a turbine to apatient, the method comprising elaborating a pressure setting for theturbine, characterised in that said pressure setting is elaborated usinga signal representative of the rotation speed of a rotating element ofthe turbine.

Preferable but not limited aspects of such a method are the following:

-   -   said signal corresponds to the rotation speed of the rotor of        the turbine,    -   the method is capable of detecting new inspiration or expiration        cycles, and to consequently adapt the level of the pressure        setting,    -   the method implements a program for detecting inspiratory cycle        using a comparison between:        -   A memorised speed value that was extrapolated using recent            values of measured speeds, and        -   An instantaneous speed actually measured,    -   the method implements a program for detecting inspiratory cycle        using a comparison between:        -   A memorised speed value as representative of a recent speed            bearing, and        -   An instantaneous speed actually measured,    -   the method implements a program for detecting inspiratory cycle        using a comparison between:        -   A memorised speed value as representative of a speed at the            end of an expiratory cycle, and        -   An instantaneous speed actually measured,    -   the method implements several programs for detecting inspiratory        cycle operating simultaneously, and elaborates a pressure        setting corresponding to a start of inspiration as soon as one        of said programs for detecting inspiratory cycle has signalled a        start of inspiration,    -   the programs(s) for detecting inspiratory cycle is (are)        associated to disabling means for a determined duration        following the start of a new expiratory cycle,    -   the means of calculation are associated to a program for        detecting expiratory cycle,    -   the method implements a program for detecting expiratory cycle,    -   said program for detecting expiratory cycle uses a comparison        between:        -   A maximum speed of the turbine memorised, corresponding to            an inspiratory cycle, and        -   An instantaneous speed actually measured,

Other aspects, purposes and advantages of the invention will appearbetter in the following description of the invention, made in referenceto the annexed drawings on which, in addition to FIGS. 1 a and 1 b whichhave already been commented in reference to the state of the art:

FIG. 2 is a schematic representation of a device according to theinvention,

FIG. 3 is a graphical characteristic of a turbine implemented in adevice according to the invention, showing for a given value of turbinerotation speed the relation between the pressure output of the turbine,and the flow generated by this turbine.

FIG. 4 includes three graphs showing a typical evolution of pressure, offlow, and of a parameter associated with the turbine of a deviceaccording to the invention, during an alternation of inspiratory andexpiratory cycles.

FIGS. 5 to 8 illustrate four modes for detecting the start of a newinspiratory cycle,

FIG. 9 illustrates a mode for detecting the start of a new expiratorycycle.

With reference now to FIG. 2, a device 20 according to the invention isshown in a schematic manner.

This device includes (as the devices of the state of the art) a turbine100 a, means 120 a allowing a patient to receive the pressurized gascoming from the turbine, and a duct 110 a for a carrying said gas fromthe turbine 100 a to the means 120 a.

Here again, the means 120 a can be a mask comprising leakage means, orinclude an expiratory valve.

The device according to the invention includes (as the device shown inFIG. 1 b) means for regulating the pressure.

Note however that in the case of the invention no flowmeter isassociated to duct 210.

Means for regulating the pressure indeed include means of calculation132 b capable of receiving from the turbine a value of a signal which ischaracteristic of the operation of said turbine, via a connection 131 b.

Means of calculation 132 b include a microprocessor and are connected toa memory, in which different parameters are memorized.

The value characteristic of the operation of the turbine is a signalcorresponding to the rotation speed of a rotating element of the turbine(e.g. its rotor).

In the remainder of this text, the signal received from the turbine willbe called “measured speed”.

To provide the means of calculation 132 b with this measured speed, aspeed sensor is integrated into the turbine. This speed sensor can befor example a Hall effect sensor.

Details will be provided hereafter concerning the different modesaccording to which the means of calculation are capable of automaticallydetecting the start of inspiratory and/or expiratory cycles, accordingto the signal received from the turbine and to different memorisedparameters.

Means of calculation 132 b are also connected to a circuit 240 forregulating the rotation speed of the turbine.

This circuit 240 receives two inputs:

Via a first connection 241, it receives a pressure setting elaborated bythe means of calculation 132 b,

Via a second connection 242, it receives a pressure measured by apressure sensor 250 on duct 110 a.

As a function of these two inputs, the circuit 240 is capable ofelaborating a rotation speed setting that is sends to turbine 100 a viaa connection 243.

This rotation speed setting is elaborated by the circuit 240 so that thepressure measured by sensor 250 reaches the value of the pressuresetting received from the means of calculation 132 b.

Note that the circuit 240 can be embodied by one or more component(s) ofthe turbine.

The device described above makes is possible to control the pressure ofthe respiratory gas carried by duct 110 a to the patient.

More precisely, a first advantage of this control is to allow theestablishment of a pressure with a desired value, corresponding to avalue of the pressure setting that is received from means of calculation132 b.

In this way, when a bearing for which a constant pressure is to bemaintained, circuit 240 is permanently active since it receives in realtime the pressure measured by sensor 250, and it constantly adapts therotation speed setting sent to the turbine in order to regulate thepressure.

And beyond this regulation of turbine rotation speed in order tomaintain pressure at a given value, the invention makes it possible todetect in real time modifications in the respiratory behaviour of thepatient, in order to trigger new inspiratory or respiratory cycles byhaving a modified pressure setting sent to the regulation circuit 240 bymeans of calculation 132 b.

To this effect, means of calculation 132 b use the measured speedreceived from the turbine.

This speed is constantly measured, and acquired at regular intervals,for example every 100 milliseconds. It is also possible to provide forcontinuous acquisition.

Note that in order to realise such a directing of the pressure settingaccording to a measured speed coming from the turbine, it is necessarythat the sum of the inertia of the device remains low enough to becompatible with a control of this pressure setting in real time.

It would in fact not be acceptable for the new pressure setting toarrive at circuit 240 while the respiratory event that led to this newsetting has occurred for a time that is too long.

In practice, the maximum acceptable delay between the respiratory eventand the elaboration of the corresponding new pressure setting is about50 to 100 milliseconds.

The inertia of the device which are likely to introduce delays in theelaboration of this pressure setting are mainly derived from:

-   -   turbine 100 a    -   duct 110 a    -   pressure sensor 250.

The inertia associated to duct 110 a and to sensor 250 are classicalpneumatic inertia, which are generally totally compatible with themaximum reaction timeframe mentioned above.

The inertia associated to the turbine must, as far as it is concerned,have a reduced value.

To this effect, a turbine with very low inertia has to be implemented inthe invention.

Such a turbine can comprise e.g.:

-   -   a portion bearing the blades having a diameter of about 44 mm        for a weight of about 5.6 g—which corresponds to an inertia of        about 90 g·cm²,    -   a rotor having also an inertia of the same order (about 90        g·cm²).

Therefore, the total inertia of the turbine remains lower than a valueof about 200 g·cm².

Below are the general principles that are the basis for exploiting ameasured speed here from the turbine in order to direct the pressuresetting of the device.

Assuming that a patient who wants to breathe provides an energy

E which is equal to DxP, with:

D: instantaneous flow of the patient,

P: instantaneous pressure of the patient,

The effort provided by the patient during an interval of time dt is thefollowing:dE/dt=(dD/dt)·(dP/dt).

Since the device must compensate for the efforts of the patient, theturbine of the device must provide over the same time interval a workthat corresponds to the effort of the patient.

The turbine of the device has a pneumatic power, which is according tothe rotation speed of this turbine:P _(turb) =f(n)

where P_(turb): pneumatic power of the turbine and

f(n): function of turbine speed.

And the pneumatic energy of the turbine is therefore of the form:E=dP _(turb) /dt=f′(n)

where

-   E: pneumatic energy,-   dP_(turb)/dt: variation in pressure over interval dt,-   f′(n): in relation to time of the function of the turbine speed.

The following balance must therefore be obtained:De/dt=df′(n)/dt=d ² f(n)dt ² =dD/dt.dP/dt.

Thus is obtained:dE/dt=f″(n)=dD.dP/dt ²

where dE: pneumatic effort,

-   f″(n): function of turbine speed resulting from f(n),-   dD: variation in patient flow,-   dP: variation in patient pressure.

FIG. 3 shows a graph characteristic of a turbine implemented in a deviceaccording to the invention.

This graph shows the relationship between the flow and the pressure ofthe turbine, for a given rotation speed.

It is thus possible to plot such a characteristic curve for eachrotation speed.

It is also possible to exploit the measurements of variation in turbinerotation speed, in order to determine, from a reference curve such asthat in FIG. 3 and calculated for a given rotation speed, thecharacteristic parameters of the turbine for a second rotation speed.

The Applicant has thus determined a law for a reference rotation speedno (corresponding in the case of the turbine used to 46000 rpm).

For this rotation speed, between the flow and the pressure, arelationship is obtained of the following type:Q=A2.Dp ² +A1.Dp+A0 with:Dp=pressure from sensor 250A2=−6.47.10⁻⁴A1=−3.45.10⁻³A0=−5.92

This reference equation corresponds to the graph in FIG. 3.

For a second rotation speed n1 that is not equal to n0, the Applicanthas established a law of transposition:T=Dp.n0/n1²andQ=(A2T2+A1t+A0).n1/n0,with:Dp=pressure from the sensor,T=rotation speed adaptation factor,Q=flow,

This law of transposition establishes a correspondence between theturbine rotation speed and the flow parameters of the respiratory gasgenerated by this turbine.

It makes it possible to determine the conditions for establishingrespiratory gas flow, and to control the operation of the device, byusing a measurement of turbine rotation speed.

Note that in the case of the invention, a measurement of flow is notexploited, but rather a measurement of turbine rotation speed.

This gives access in particular to the following advantages:

-   -   to be free from the presence of a flowmeter,    -   to work in real time in relation to the operation of the        operation (indeed, in known devices in which the control is        performed using a flow measurement, a certain time is needed for        a modification in the turbine operating conditions translate        into a difference in flow at the level of the flowmeter—and this        “certain time” is largely greater than the limits mentioned        above, making a “real time” control impossible).

It is specified regarding this aspect that the device and processaccording to the invention use only a measurement of speed of theturbine.

In this respect, the invention totally differs from known devices suchas the one described in patent EP 656 216.

This patent mentions turbine motor speed as a parameter which can beused in some way to control the operation of the device.

This patent further states that modifications of the airflow which aredue to the respiration of the patient will alter parameters such as theturbine motor's speed and/or current.

It further explains that it proposes to use specific signals to detectthe points at which the patient starts to inhale and exhale.

As exposed column 3 lines 37-39 of this patent, these specific signalscan be derived:

-   -   Either from the motor speed and power measurements (first        option), or    -   From the spill valve position and power measurements (second        option).

In the first option, motor speed is thus used in some way for detectingthe beginning/end of respiratory cycles. But it is to be noted thatmotor speed is in no way used by itself, as a single control parameter.This parameter is indeed systematically used in combination with powermeasurements.

This is understandable, since turbines known at the priority date of EP656 216 had quite an important inertia—in the order of 1500 g·cm² atleast.

For such turbines, a change in the airflow conditions would not changeturbine speed before some inertia time (greater than the limit mentionedabove).

Thus, in order to detect such airflow changes in “real time” (or atleast as fast as possible, which is of course desired), the meremonitoring and exploitation of turbine speed would in any event not havebeen appropriate.

Therefore, such known devices have to use as a main parameter notturbine speed but motor power, in order to detect airflow changes asfast as possible.

In turbines such as the ones known at the priority date of this patent,this parameter “motor power” will indeed vary much faster than theturbine speed, which can change only after some time because of thelarge inertia of the turbine.

It should be further noted that the systematic exploitation of motorpower in the case of EP 656 216 makes it necessary to filter the signalsbased on power.

This is not the case for the present invention, turbine speed in itselfrequires no filtering, and can be exploited per se.

The invention also strongly differs from earlier disclosures such as theone of EP 505 232, which discloses the use of a signal which is thecontrol signal sent to the turbine itself.

The idea which is exploited in this document is the fact that whenairflow conditions change, the control loop which sends said controlsignal to the turbine will adapt very quickly its control signal inreaction to the change in pressure detected in the inspiration duct.

Thus, this other prior art patent does not teach or even suggest usingdirectly the turbine speed to detect new respiratory cycles and controlthe operation of the turbine.

Furthermore, here again, the signal used for controlling the turbinedoes not provide a direct indication of the airflow conditions: thesignal indeed comprises electrical noise which must be filtered.

And in any event, here again at the priority date of the patent theturbines used in respiratory devices had a large inertia which made thedevice incompatible with a real time control of the turbine based onturbine speed measurements.

Finally, it is specified that the prior art discloses in US 2003/0015200a respiratory devices which use a speed signal for its operation.

This document indeed mentions a use of turbine speed.

But the use disclosed by this document is very different from the usemade in the present invention: this prior art document concerns a veryspecific device with two gas sources (a pressurized gas source, and adepressurized source).

The general architecture of the device of this document is thuscompletely different from the architecture of the device of the presentinvention, which comprises only one source for the respiratory gas.

And not surprisingly, this fundamental difference in the structure ofthe device is also associated with a difference in the exploitation ofthe turbine speed: in the case of US 2003/0015200 turbine speed is usedfor activating a three-way valve 211, not for controlling a turbine.

In fact, US 2003/0015200 is not concerned by the control of a turbine.

In the case of this document the control of the airflow conditions isindeed sought through this valve 211, which selectively connects thepatient to one of the two gas sources 234, 236.

As said above, the device according to the invention is capable ofelaborating in real time (i.e. with a timeframe less than the maximumtimeframe mentioned above) pressure settings corresponding to newinspiratory and/or expiratory cycles.

More precisely, means of calculation 230 are associated with severalprograms for detecting inspiratory cycles being able to operatesimultaneously.

Each one of these programs for detecting inspiratory cycles follows inreal time the changes in certain parameters of the respiratory activityof the patient, and is capable of triggering a new inspiratory cyclewhen conditions that are proper to the program are met.

And when the different programs operate simultaneously, as soon as theconditions corresponding to a new inspiratory cycle for one of theprograms are met, the means of calculation 230 elaborate a pressuresetting corresponding to a new inspiratory cycle and transmit thispressure setting to the turbine.

Below is described the different detection modes for a new inspiratorycycle, corresponding to these different programs.

Note that it is possible to implement only one of these programs, or toonly implement certain ones simultaneously.

It is also possible to implement them all simultaneously as mentionedabove.

Before describing in greater detail the different detection modes for anew inspiratory cycle, in reference to FIG. 4 the typical change inseveral parameters during a succession of inspiratory and expiratorycycles is reminded.

The two curves in the upper portion of this figure show respectively thechange in the pressure in the duct carrying the respirator gas and inthe corresponding gas flow.

The different cycles are marked in the lower portion of the figure (Ifor the inspiratory cycle, E for the expiratory cycle).

The lower curve shows the change during the same succession of cycles inthe turbine speed signal.

Note that this curve varies according to the turbines used, the curveshown in FIG. 4 (and which will be used in the rest of this text)corresponding to a turbine of which the inertia corresponds to thevalues mentioned above.

The lower curve shows the fact that turbine rotation speed in the deviceis notconstant.

On the contrary, this speed is higher during the inspiratory phases, andlower during the expiratory phases.

Therefore, the change in this rotation speed as a function of timecontains information associated to the respiratory behaviour of thepatient.

And as we shall see, the invention uses this information to detect newinspiratory or expiratory cycles, and consequently modifiy the pressuresetting which is elaborated by means of calculation 230, and send to theregulation circuit 240.

It is to be noted that in all the FIGS. 5 to 8, which show differentmodes for triggering inspiratory and/or expiratory cycles, the valuesmeasured in real time are represented with white points, while blackpoints represent memorised values.

In reference now to FIG. 5 the triggering of an inspiratory cycleaccording to a first mode, called “fast effort detection”, is described.

As we shall see, this detection mode uses a comparison between

-   -   an instantaneous speed actually measured by the speed sensor of        the turbine (it is reminded that in this text “speed” designates        the rotation speed of a rotating element of the        turbine—typically its rotor),    -   and parameters memorised in the memory mentioned which has been        above and which is associated to means of calculation 230.

More precisely, the variations in this rotation speed are used in thedifferent modes for detecting the start of inspiratory cycles, but alsoin the detection of the start of expiratory cycles.

Returning to the description of this first mode for detecting the startof an inspiratory cycle, the program associated with this first modeconstantly evaluates an extrapolation f the speed according to thelatest measured speeds.

This extrapolation can for example be performed on the basis of the lasttwo speeds actually measured.

The value extrapolated in this way is memorised in the memory associatedto the means of calculation 230. It is represented by a black dot on thegraph in FIG. 5.

At every given instant, an extrapolation is available that correspondsto an expected speed at the next speed measurement.

And during this next speed measurement, the program compares theextrapolated speed value with the value of speed actually measured.

If the difference between these two speed values is greater than a giventhreshold, the program concludes to an initiation of a new respiratorycycle.

In this case, means of calculation 230 consequently elaborate a newadapted pressure setting.

This threshold can be for example 2200 points, with “points” being aunit that is representative of the turbine rotation speed.

Note that this detection mode can—as with all the other modes fordetecting the start of a new inspiratory cycle—be disabled for a giventimeframe following the start of a new expiration (with expiratorycycles being initiated in the manner that will be described further onin this text).

An auto-triggering of the inspiratory cycle by error is thereforeavoided in the case for example of an expiratory valve rebounding (whichcorresponds to a negative variation then a positive one for pressure atthe start of expiration).

The timeframe for disabling this triggering of a new inspiratory cycle(i.e. the time during which the triggering is disabled) can be forexample about 300 milliseconds.

FIG. 6 shows a second mode for detecting a new inspiratory cycle.

This mode is activated when operating stability in the turbine isobserved for a duration that is greater than a given minimum value, saidgiven value being memorised in the memory of the means of calculation230 as all the operating parameters for the device.

Said given minimum value can be 300 milliseconds, for example.

Note that “stability” is defined as the operating conditions thatcorrespond to speed variations contained within given percentages ofvariation (corresponding to a triggering threshold).

By way of example, for a nominal turbine rotation speed of about 40,000to 6,0000 rpm, these given variation percentages correspond to values ofabout 100 to 400 rpm, depending on the pressure present in duct 210.

Note that the lower this pressure is, the more it is possible totolerate large variations in speed, while still considering that thestate is “stable”.

Thus, in a range of low pressures, the range of speed variationscorresponding to “stable” operating conditions is enlarged, andapproaches a maximum range of 400 rev/min more or less.

When this mode is in this way activated, the average value of rotationspeed corresponding to the range of stability is memorised in the memoryassociated to the means of calculation 230.

Since turbine rotation speed is always measured in real time, a newinspiratory cycle is triggered by the means of calculation when theabsolute value of the difference between the measured speed and saidmemorised stability value is greater than the triggering threshold.

This triggering threshold can be for example 1800 points. We have seenthat its value can further depend on the pressure measured

FIG. 6 thus corresponds to triggering an inspiratory cycle in a modecalled “detection of substantial effort after stability”.

FIG. 7 corresponds to a similar mode, operating on the basis ofdifferent values.

In this case, the mode is called “detection of substantial effort afterprolonged stability”.

In this case, the duration of stability from which the value of speed ismemorised is 500 milliseconds, not 300 milliseconds. It is in any eventlonger than the corresponding value for the preceding mode.

And the triggering threshold is lower (typically 1500 points, instead of1800).

FIG. 8 shows another mode for triggering an inspiratory cycle, called“cycle to cycle detection of effort”.

This mode is implemented by memorising the value of measured rotationspeed at the end of the previous expiratory cycle.

This memorised value in association with means of calculation 230 willserve as a reference for triggering not the inspiratory cycle thatimmediately follows the acquisition of this memorised value, but of theinspiratory cycle that will follow yet.

The program corresponding to this mode triggers a new inspiratory cyclewhen the absolute value of the difference between the measured speedvalue and this memorised value is greater than a given triggeringthreshold.

This triggering threshold can be for example about 2200 to 2500 points(note that the curves are not strictly to scale—in particular withregards to the triggering thresholds).

In reference now to FIG. 9, a mode for triggering expiratory cycles isshown.

To implement this triggering, the device memorises the maximum value forturbine rotation speed (this maximum is produced during inspiratorycycles).

A new maximum can therefore be memorised at each inspiratory cycle.

It is also possible to only memorise a new maximum value only every Ninspiratory cycles, N being able to be set freely.

When the speed measured in real time goes down to a value thatrepresents a determined proportion of this maximum, means of calculation230 provoke the triggering of a new expiratory cycle, and elaborate tothis effect an adapted pressure setting.

The proportion in question can be for example 70% of the memorisedmaximum.

Note that this triggering of a new expiratory cycle can be disabled fora given duration (for example 200 milliseconds) after the occurrence ofthe maximum rotation speed in the inspiratory cycle.

It thus appears that the invention makes it possible to regulate in realtime the operation of a turbine, by following the rotation speed of thisturbine and the pressure of the gas carried to the patient.

And the invention also makes possible, using the monitoring of theturbine rotation speed, to trigger new inspiratory and/or expiratorycycles, by modifying a pressure setting sent to the turbine.

It is to be noted that the invention makes it possible to avoid using aflowmeter, and the limitations that are associated with such acomponent.

And as mentioned above, this invention also makes it possible toactually control the device in real time, which improves patientcomfort.

1. A breathing assistance device, comprising: a turbine to generate aflow of pressurised respiratory gas, said turbine having an inertia lessthan about 200 g·cm², a duct adapted to carry the pressurised gas to apatient, and control means for controlling gas pressure capable ofcomputing a pressure setting for the turbine, wherein the turbine isconnected to a speed sensor capable of acquiring a signal correspondingto a rotation speed of a rotating element of the turbine, and thecontrol means includes means of calculation connected to said speedsensor to compute the pressure setting using only said speed signal andsend the pressure setting to the turbine, said means of calculationbeing adapted to detect new inspiratory or expiratory cycles using onlysaid speed signal, and consequently adapting a level of said pressuresetting.
 2. A device as claimed in claim 1, wherein said speed sensorimplements a Hall effect sensor.
 3. A device as claimed in claim 1,wherein said speed sensor is a sensor capable of acquiring a turbinespeed signal directly connected to the rotation speed of the rotatingelement of the turbine.
 4. A device as claimed in claim 1, wherein themeans of calculation computes the pressure setting according tovariations in speed.
 5. A device as claimed in claim 1, furthercomprising a program for detecting an inspiratory cycle using acomparison between: a speed value stored in a memory of the device, saidspeed value being extrapolated using recent values of measured speeds,and an actually measured instantaneous speed.
 6. A device as claimed inclaim 1, further comprising a program for detecting an inspiratory cycleusing a comparison between: a speed value stored in a memory of thedevice, said speed value being representative of a recent speed bearing,and an actually measured instantaneous speed.
 7. A device as claimed inclaim 5, further comprising a program for detecting an inspiratory cycleusing a comparison between: a speed value stored in a memory of thedevice, said speed value being representative of a speed at an end of anexpiratory cycle, and an actually measured instantaneous speed.
 8. Adevice as claimed in claim 5, further comprising several programs fordetecting an inspiratory cycle operating simultaneously, and is capableof computing a pressure setting corresponding to a start of inspiratorycycle as soon as one of said programs for detecting the inspiratorycycle has signalled a start of inspiration.
 9. A device as claimed inclaim 5, wherein the program for detecting the inspiratory cycle isconfigured to be disabled for a determined duration following a start ofa new expiratory cycle.
 10. A device as claimed in claim 1, furthercomprising a program for detecting an expiratory cycle.
 11. A device asclaimed in claim 10, wherein said program for detecting the expiratorycycle uses a comparison between: a maximum turbine speed stored in amemory of the device and corresponding to an inspiratory cycle, and anactually measured instantaneous speed.
 12. A device as claimed in claim1, wherein said means of calculation includes a microprocessor connectedto the speed sensor and to a turbine pressure setting input.
 13. Adevice as claimed in claim 1, wherein the device further includes apressure-regulating loop comprising: a pressure sensor on the duct, anda circuit receiving the pressure setting coming from the means ofcalculation as well as a pressure measured by the pressure sensor, saidcircuit being capable of computing an instantaneous rotation speedsetting for the turbine, said circuit being connected to a turbine speedsetting input.
 14. A method for regulating a pressure of a respiratorygas delivered by a turbine to a patient, the method comprising:providing, with a speed sensor, a signal representative of a rotationspeed of a rotating element of the turbine, said turbine having aninertia less than about 200 g·cm²; and computing, with a processor, apressure setting for the turbine based only on the signal representativeof the rotation speed, said method being adapted to detect newinspiratory or expiratory cycles using only said signal, and ofconsequently adapting a level of the pressure setting with theprocessor.
 15. A method as claimed in claim 14, wherein said signalcorresponds to the rotation speed of the turbine rotor.
 16. A method asclaimed in claim 14, wherein the method implements a program fordetecting an inspiratory cycle using a comparison between: a speed valuestored in a memory, said speed value being extrapolated from recentvalues of measured speeds, and an actually measured instantaneous speed.17. A method as claimed in claim 14, wherein the method implements aprogram for detecting an inspiratory cycle using a comparison between: aspeed value stored in a memory, said speed value being representative ofa recent speed bearing, and an actually measured instantaneous speed.18. A method as claimed in claim 14, wherein the method implements aprogram for detecting inspiratory cycles using a comparison between: aspeed value stored in a memory, said speed value being representative ofa speed at the end of an expiratory cycle, and an actually measuredinstantaneous speed.
 19. A method as claimed in claim 16, wherein themethod implements several programs for detecting inspiratory cyclesoperating simultaneously, and computes the pressure settingcorresponding to an inspiratory flow as soon as one of said programs fordetecting the inspiratory cycles has signalled the start of inspiration.20. A method as claimed in claim 16, wherein the program for detectinginspiratory cycles is disabled during a determined duration followingthe start of a new expiratory cycle.
 21. A method as claimed in claim14, wherein the method implements a program for detecting expiratorycycles.
 22. A method as claimed in claim 21, wherein said program fordetecting expiratory cycle uses a comparison between: a maximum turbinespeed stored in a memory and corresponding to an inspiratory cycle, andan actually measured instantaneous speed.